This invention concerns a shading data accepting system in an image data reader for reading image data from digital copiers, facsimiles, printers or other hardware.
Conventional shading correction at white levels will now be explained with reference to FIGS. 1(A-C).
A contact type of dot-sequential line sensor 50 is built up of a zigzag array of sensor elements connected with each other, each of said elements comprising five chips, for instance. The read-out video signal is placed under gain and offset controls by a gain control circuit 51 and an offset control circuit 52, respectively, and converted into digital data by an A/D converter 53, which is in turn separated by a multiplexer 54 into respective R, G and B signals. Outputs of a buffer 55 are converted into density data by logarithmic transformation corrected for shading by a subtraction circuit 57 and a memory 58, and placed under gray balance control by an END conversion table 60 for output to an IPS.
The goal of shading correction is to compensate for variations due to the illuminating characteristics of light sources and changes with year and non-uniformity of light source, variations in optical systems resulting from stains oh reflectors, variations in the sensitivity of sensors to pixels, etc. At the beginning of scanning, a white reference plate 41 located on the lead edge side is read out, as typically shown in FIG. 1b, and the readout data is stored in the shading memory 58 after logarithmic transformation. Acceptance of the white reference plate data takes place by sensing that a reader is lying just under the white reference plate 41 by a position sensor 59 and triggering the detected signal to allow the memory 58 to accept the read-out data. Then, the original document is sequentially read out by the line sensor 50 and, after logarithmic transformation, the data stored in the memory 58 is subtracted from the data read at the subtraction circuit for the purpose of shading correction.
Ideally, the light source illuminating the original document should be as large in terms of the quantity of light as possible with its spectral characteristics being flat. Indeed, however, there is unavailable such a light source; that is, currently available light sources give out R, G and B components at considerably different ratios. At one end is a daylight fluorescent lamp whose R to B ratio is relatively small and whose characteristics are flat. Although the fluorescent lamp has the general merits of being less in power consumption, having an R:G:B ratio approximate to 1:1:1 and being more enhanced than a halogen lamp in terms of blue, a heater is required for temperature control, because its quantity of light is considerably variable. Once copying has been initiated, however, that lamp remains lit, so that its temperature rises, only to decrease in the quantity of light. This also holds at a low room temperature; its temperature drops under the influence of air vagaries resulting from scanning, again only to decrease in the quantity of light. Thus, not only is some difficulty involved in keeping the fluorescent lamp at the best temperature (about 45.degree. C.) at which the maximum quantity of light is obtained, but the quantity of light is variable as well. A halogen lamp, on the other hand, is well stabilized in terms of the quantity of light but is far from satisfactory in terms of the R:G:B ratio, esp., in terms of the proportion of B. Accordingly, when it is intended to amplify the components R, G and B separately so as to put the signal strengths of such colors in order, it is required to separate the light into three colors at the analog signal stage and then amplify them separately for compositing. This is particularly true of the use of a contact type of dot-sequential sensor. Even when five chips are used as the line sensor, fifteen (3.times.5) amplifiers are needed, posing a problem in that the circuit costs much.
Using a rod lens array, the conventional contact type of dot-sequential line sensor has the advantages of being capable of reducing the power of the light source due to its high resolution and brightness and being made compact. However, it has a grave problem in connection with a zigzag array of line sensor chips, correction of a time lag corresponding to a positional deviation between adjacent chips, correction of chip joints, inter-chip differences in characteristics, etc. Another problem arises from the fact that the rod lens array has a very small focal depth;. even a slight separation of the original document from the platen surface renders the image out of focus. For these reasons, such a demagnification type of line-sequential line sensor as illustrated in FIGS. 2(A and B) has been proposed in the art.
As typically shown in FIG. 2a, the demagnification type of line-sequential line sensor is designed to focus image-bearing light from a platen surface 40 onto a CCD line sensor 60 through a demagnification type of optical system 61. As can be best seen from FIG. 2b, this line sensor 60 is built up of three R, G and B sensor elements 60a, 60b and 60c located at given intervals. The sensor elements are each formed of a single chip so that R, G and B signals can be obtained separately without offering such inter-chip variations as found in the case of the contact type of dot-sequential line sensor. This enables the colors to be placed under separate gain controls, thus making it possible to use illuminating hardware ill-balanced between R, G and B, for instance, a halogen lamp. Another advantage of using the demagnification type of optical system for the purpose of image formation is that the focal depth is large enough that the resulting image can be well in focus. In the case of the demagnification type of optical system, however, such a large focal depth itself becomes a problem. In other words, when the white reference plate is positioned on the platen surface, as typically shown in FIG. 1b, dust present on the platen surface, even if slight, forms an image, which is then read-out to add a glitch to the white reference data, so that the image signal read-out with pixels corresponding to the dust-carrying portion of the white reference cannot precisely be corrected for shading, giving a defective or streaked output image.
The white reference plate 41, when placed on the platen glass, is usually less stained but, when used with an automatic document feeder (ADF), is likely to be stained by dust entering between it and a movable member 42 located at the left end of the platen so as to allow for smooth discharge of the original document, as illustrated in FIG. 1c. This dust, if remains deposited onto the white reference plate, is particularly detrimental to image quality.
When using a conventional fluorescent lamp, there is a variation in the quantity of light between when the shading data is gleaned and when the image is read, because its quantity of light is unstable. This poses a problem in that "image fogging" or "discrete imaging" occurs as a result of shading correction.
Further, because of being placed on the lead edge side, the white reference plate is read to update the white reference data whenever scanning is initiated. In the mode of copying a small document placed at the end opposite to the register end or copying it continuously on an enlarged scale, however, it is impossible to update the white reference data, because the image reader is kept from returning to the white reference plate whenever scanning is initiated so as to increase the productivity of copies; because of the quantity of light varying in the meantime, the highlight area of the image is fogged or made discrete.